1. Field of the Invention
This invention relates to an underground, contained pipe assembly.
2. Description of the Prior Art
Underground tank systems are defined as any system that has 10% or more of its overall volume underground, including associated piping. Such systems are regulated by legislation which require the use of secondary containment with interstitial monitoring for systems that contain or transport hazardous chemicals or substances. The provision of secondary containment is optional for systems that carry or transport oil, gasoline or other petroleum based products, but in any case, the use of corrosion resistant materials is mandatory for new systems. The concept of double containment piping provides the additional degree of safety for pipes that are designed to transport hazardous, toxic or extremely corrosive chemicals. There have been many products developed to date, but the products that have been developed have not completely addressed some of the unique problems that arise when a tank or pipe is placed within another tank or pipe.
One such problem is the ease of maintaining, modifying or repairing such systems. To date, most underground storage tank double wall systems have been installed without having an ability to have ready access to the systems in the event of an upset condition. Therefore, if a repair or modification is to be made, the original system manufacturer/contractor must be called in to facilitate the repair. In many designs, a repair of a certain section would not be feasible or even possible due to the location of the system with respect to adjacent equipment or building parts. In any case, the fact that a tank and/or piping system is directly buried results in an expensive excavation in the event of a repair. What would be very helpful to most facility owners is the ability to have a system that is readily accessible. A system capable of being readily accessible would give the facility engineer the ability to have the tank or pipe section requiring modification or repair capable of being modified without a major excavation and disruption to the business. In some instances, a faulty tank or pipe section could be sent to a shop for repair, and then readily placed back into service.
Another such problem has to do with the ability of associated underground piping systems to withstand the effects of inner and outer piping and components that are subjected to different amounts of thermal expansion and contraction. It is the norm, rather than the exception, that the inner and outer pipes of any double containment piping system are subjected to different amounts of thermal expansion. This situation may arise in several different ways. The most common way involves the situation whereby a hot fluid is transported through the inner pipe. Under this circumstance, the surrounding soil (external that is to the outside diameter of the secondary containment piping), is normally at a lower temperature than the hot fluid. Since there is either an insulating dead air space between the two pipes, or other insulating material (dead air space is always the better form of insulation), the inner piping temperature becomes close to that of the fluid, while the outside piping remains closer to that of the external ambient environment. Therefore, the materials normally grow to different lengths due to their being at different temperatures. Even if the outer pipe were to be at similar temperatures, the surrounding soil would tend to keep the secondary containment piping restrained, if it were properly buried. When this does occur, there are thermal strains that are imposed on interconnecting parts, or interstitial supports that create a contact point between the inner and outer piping, and at fitting locations. The most obvious place where there is a problem potential is at the interconnecting points. However, any place where loads can be transmitted back and forth between the two pipe systems, or anywhere that a fitting can become excessively strained can result in a problem. In the prior art, all interconnecting parts designed for this situation are constructed of a singular material, with such a design that the residual stresses and subsequent strain on the materials can lead to a failure of such parts.
Another problem of existing systems has to do with the ability to combine different inner and outer materials in an efficient manner. Different materials can mean materials that are of a different class (such as metallic-thermoplastic, metallic-reinforced thermosetting plastic, or thermoplastic-reinforced thermosetting plastic), or materials that are within the same class but constitute a different material (such as within the thermoplastic family, combining a fluoropolymer within a polyolefin, or a polyolefin within another polyolefin). The reason that it is desirable to combine materials typically has to do with economics. In other words, it is desirable in many situations to combine an expensive material that is capable of handling a chemical on a full time basis within a less expensive material capable of withstanding the corrosive effects of a chemical for a limited period of time. Another major economic reason has to do with the use of a material for the outside piping capable of withstanding the corrosive effects of soil, thus eliminating the need for expensive coatings, cathodic protection, etc. An application of this would be in combining a metallic material within a non-metallic outside material (such as a thermoplastic) for the reasons just described.